Background
Lung cancer, as the most common malignancy, is the leading cause of cancer-associated mortality [
1]. The prognosis of lung cancer patients is very poor and the five year survival rate is no more than 10% [
2,
3]. The primary histological subtype is the non-small cell lung cancer (NSCLC) constituting over 80% of lung cancer [
4]. As the two major subtypes of NSCLC, adenocarcinoma (ADC) and squamous cell carcinoma (SCC) have been considered to be similar in postsurgical prognosis and chemotherapeutic response for very long time. However, a number of recent studies have found that ADC and SCC have a variety of differences not only in postsurgical prognosis and chemotherapeutic response, but also in molecular characteristics and therapeutic strategies [
5‐
9]. These studies demonstrate that efficacy of therapy options varies for different subtype patients, highlighting the importance of characterizing molecular abnormalities in different tumor-subtypes [
10‐
14]. Thus, identification of novel gene and clarification of its specific function and clinical value are crucial for developing new and special therapeutic approach for particular subtype patients of NSCLC.
SOX30, as an important SOX (sex determining region Y-box) family factor, has been reported to be involved in spermatogenesis [
15‐
18]. For many years, the roles of SOX30 on tumorigenesis are completely unknown. Recently, SOX30 has been identified as a key participant in tumorigenesis of several type tumors including colon cancer and lung cancer [
19‐
23]. However, the value and precise role of SOX30 in different subtype patients of NSCLC remain largely unexplored.
In our present study, we assessed an unknown clinical significance, specific tumor suppressor role and molecular mechanism of SOX30 in different subtype NSCLC using clinical association analysis, prognostic analysis and differentially expressed cell models. Our findings demonstrate that SOX30 is a special tumor-metastasis suppressor and early-stage favorable prognostic factor by directly repressing Wnt/β-catenin (CTNNB1) signaling activity only in ADC patients, providing novel insights into and specifically potential molecular-targeted strategy for NSCLC therapy.
Methods
Cell lines
The human ADC cell lines: A549 and SPC-A-1 and SCC cell lines: H520 and H2170 were obtained from the Cell Bank of Chinese Academy of Science (CBCAS, Shanghai, China) and the American Type Culture Collection (ATCC, Manassas, VA, USA), cultured in F12K (Sigma, St. Louis, MO, USA) or PMI-1640 (HyClone, Logan, UT, USA) media supplemented with 10% fetal bovine serum (FBS) (Gibco, Carlsbad, CA, USA), and incubated at 37 °C in 5% CO2 cell incubator.
Patient samples
A total of 540 NSCLC (275 ADC, 231 SCC and 34 large cell carcinoma [LCC]) patients undergone surgical resection between 2004 and 2014 were recruited from Daping Hospital or Southwest Hospital Affiliated to Third Military Medical University in Chongqing, China. The clinico-pathologic information was retrieved from the electronic medical records of patients, which includes age, gender, histological type, histological grade, clinical stage, tumor size, tumor location, lymph node, tumor diameter, tumor metastasis and Overall survival (OS). OS was defined as the period from surgery to death or the last observation. The clinico-pathologic parameters also include the expression of CTNNB1. The study was approved by the ethics committee of Daping Hospital and Southwest Hospital. The informed consent was signed by the patients participating in the study. All experiments were performed in accordance with the approved guidelines of the Third Military Medical University.
Tissue microarray generation and immunohistochemical (IHC) analysis
The tissue microarray was generated and IHC analysis was performed as a previous study [
20]. IHC staining was performed using the antibody against SOX30 (1:100, Santa Cruz Biotechnology, Germany), and was evaluated and defined as positive when immunoreactivity ≥5%. Positive percentage of cell staining was classified into 5 categories: < 5% positive cells as 0 score; 5 to 25% as 1 score; 26 to 50% as 2 score; 51 to 75% as 3 score and ≥ 76% as 4 score. Intensity of cell staining was graded as negative for 0 score, weak for 1 score, moderate for 2 score and strong for 3 score. SOX30 expression levels were calculated by product of category for the positive percentage and grade for the intensity of staining [
24,
25]. The range of the calculation for SOX30 expression was therefore 0–12 score. The expression of CTNNB1 was measured by IHC staining using the antibody against CTNNB1 (1:150, Santa Cruz Biotechnology). IHC staining of CTNNB1 was then evaluated and defined as positive when immunoreactivity ≥5%, otherwise it was identified as negative. All biopsy tissues of IHC staining were independently reviewed by two blinded pathologists.
RNA isolation and RT-PCR
Total RNA was extracted from A549, SPC-A-1, H520 and H2170 cells. Approximately 3.0 μg of RNA was treated with DNase I (Invitrogen Preservation, Carlsbad, CA, USA) to eliminate the genomic DNA, and then was reverse-transcribed to cDNA. RT-PCR (Reverse transcription polymerase chain reaction) was performed as previously described [
19]. The following primers were used in this study. SOX30 forward primer (5′-3′): GAT GTC CCG CTC ACC GTG TTG C, SOX30 reverse primer (5′-3′): GAC AGG GCT TGG GCT CTG GAC T; β-actin forward primer (5′-3′): GAG CTA CGA GCT GCC TGA CGG, β-actin reverse primer (5′-3′): CCT AGA AGC ATT TGC GGT GG.
Western blotting analysis
Western blotting (WB) analysis was performed as described previously [
19]. The primary antibodies used this study were: SOX30 rabbit polyclonal antibody (1:800; Santa Cruz Biotechnology, sc-20,104), CTNNB1 mouse monoclonal antibody (1:800; Santa Cruz Biotechnology, sc-7963). Secondary antibodies were horseradish peroxidase (HRP)-conjugated (1:4000, Jackson, PA, USA).
Plasmid construction and cell transfection
The plasmid construction and cell transfection were performed as described previously [
19]. Briefly, the human SOX30 plasmid was constructed by synthesis, subcloned into pIRES2-EGFP vector (Invitrogen) and was validated by sequencing. The plasmids were transfected into A549, SPC-A-1, H520 and H2170 cells using ViaFect Transfection Reagent (Promega). The stably transfected cells were screened under G418 (Calbiochem, CA, USA), and the cell clones were obtained by cylinder method.
Migration and invasion assays
The migration and invasion activities of cancer cells were evaluated using transwell assays in 24-well plates (8 μm, Corning, Acton, MA, USA) to assess the cell migration (without matrigel) and invasion (with matrigel). A549, SPC-A-1, H520 and H2170 cells stably transfected with pIRES2-EGFP-SOX30 or pIRES2-EGFP empty vector in serum-free medium were seeded into upper well of the chamber at 2 × 104 cells/well. Lower well of the chamber contained fully media supplemented with 10% FBS. The migrated cells at the lower chamber were stained with 0.1% crystal violet, and counted under an inverted microscope after 12 h for migration or 18 h for invasion. All experiments were repeated thrice in triplicate for each.
Gene ontology analysis
Gene ontology (GO) analyses were performed using DAVID 6.7 (
https://david.ncifcrf.gov/) [
26,
27]. The genes associated with SOX30 were selected for analysis when the
p-value of correlation less than 0.05. The corresponding GO terms and pathways were enriched in these related genes.
Network analysis
The PPINs, human protein-protein interaction networks, were first downloaded from the Human Protein Reference Database. The igraph package of statistical language R was used to analyze the functional profiling. The network of visualization and Cytoscape were applied to find the putative target genes. The Markov cluster algorithm was performed to identify the closely connected modules within the networks.
TOP/FOP-flash assays and luciferase reporter assays
The ADC cell line (SPC-A-1) and SCC cell lines (H520 and H2170) were transfected with CTNNB1-promoter, pIRES2-EGFP empty vector, pIRES2-EGFP-SOX30 and TOP-Flash with wild-type TCF/LEF binding sites/FOP-Flash with mutated TCF/LEF binding sites plasmids which purchased from Addgene (12,456 and 12,457, Cambridge, MA, USA). Luciferase reporter assays were performed using the fluorescence microplate reader measurement system Varioskan LUX (Thermo Fisher, Waltham, MA, USA) and a Dual-luciferase reporter kit (Promega). The Renilla reniformis luciferase reporter was selected as an internal control. The luciferase activities of luciferase reporter assay were analyzed at 36 h after transfection. All the experiments were repeated thrice.
Chromatin-immunoprecipitation RT-PCR assay
Chromatin-immunoprecipitation (ChIP) assay was performed according to the manufacturer’s protocol of the ChIP assay kit (Cell Signaling Technology). Briefly, 5 × 106 SPC-A-1, H520 and H2170 transfected cells were fixed in 1% formaldehyde. The fixed cells were digested with micrococcal nuclease and chromatin immunoprecipitated after analysis of chromatin digestion. Then the samples were eluted of chromatin and purified DNA. The immunoprecipitated DNA and the input DNA were used as the templates for RT-PCR using the following primers. CTNNB1-p forward primer 2 (5′-3′): TCT TAA CCA ATT TCA AGA GTG CCT; CTNNB1-p reverse primer 2 (5′-3′): GGC GTT TTC AGG TAC TGA TTC C.
Statistical analysis
Statistical analyses were performed using SPSS 13.0 software (SPSS, Inc., Chicago, IL, USA). The expression was categorized as high or low according to the median score. Survival analyses were calculated by Kaplan-Meier methods. Cox regression was used for multivariate analysis of prognostic predictors. The differences between two or three groups were analyzed using Chi-square test, Student’s t-test or One-way ANOVA. The p-values of less than 0.05 were considered statistically significant.
Discussion
The high-mortality of NSCLC patients is greatly due to distant metastasis [
28,
29]. Different subtypes of NSCLC usually show different molecular events of metastases and different responses of therapeutic strategy, which highlights the importance and urgency of characterizing molecular abnormalities in different tumor-types to search for novel special therapeutic targets and prolong the survival of cancer patients [
10‐
14]. In the present study, clinical analyses reveal that SOX30 is significantly associated with metastasis of ADC patients, but not associated with metastasis of SCC patients. Functional analyses indicate that SOX30 strongly inhibits tumor cell metastasis in ADC cell lines, but not affects tumor cell metastasis in SCC cell lines, which is consistent with different association of SOX30 with metastasis in ADC and SCC. Mechanistically, SOX30 is negatively associated with β-catenin expression, a key metastatic gene, only in ADC, and SOX30 indeed inhibits Wnt/CTNNB1-signaling pathway by direct binding to CTNNB1 promoter only in ADC. These data indicate that SOX30 plays a specific role on tumor metastasis via directly transcriptional repression of CTNNB1 in ADC patients.
In the clinical, more than 60% of NSCLC patients are diagnosed at advanced stage. The five-year survival rate of the patients at advanced stage is less than 10%, whereas it is greater than 70% of the patients at clinical stage I [
30,
31], indicating that identification of prognostic biomarkers for early-stage is of great importance to prolong NSCLC patient’s survival. In our present study, Kaplan-Meier and Cox regression analyses revealed that SOX30 expression was significantly correlated with ADC patient’s survival, especially with OS of the ADC patients at stage I. The elevated SOX30 expression inhibited tumor-metastasis and represented as a favorable prognostic biomarker of the ADC patients at stage I. These data suggest that SOX30 serves as a new favorable early-stage prognostic candidate for ADC patients, which can be used to accurately predict clinical outcomes of ADC patients.
In our previous study, we have also found that SOX30 expression is correlated with histological types of NSCLC, and is a favorable prognostic factor in ADC patients (
n = 150) but not in SCC patients (
n = 70) [
20]. In the present study, we further corroborated these results in a new larger cohort (
n = 275 for ADC and
n = 231 for SCC). In previous study, a better OS for the ADC patients with SOX30-high expression was seen only in the subset of stage II not in stage I [
20], which seemed to be very confusing and might be due to the very small sample size. In the new cohort with larger sample size of our present study, SOX30 expression is also associated with the OS of ADC patients at clinical stage II using univariate analysis (
p = 0.029) but not using multivariate analysis (
p = 0.668). Most importantly, the difference in our present study from previous study is that SOX30 expression is obviously associated with the OS of ADC patients at clinical stage I (Kaplan-Meier analysis,
p = 0.000; Cox-Regression analysis,
p = 0.011). These data reveal that SOX30 is an early-stage favorable prognostic biomarker for ADC patients in fact.
To ensure SOX30 is indeed an early-stage prognostic marker for ADC patients, we further determined the reliability of the result that SOX30 is a favorable early stage prognostic factor for ADC carefully. As SOX30 was poorly expressed in the patients at late stage and the expression was categorized as high or low according to the median score of the patients at all clinical stages, the number of SOX30-high expression patients at advanced stage is only 12, which is really small. Unfortunately, it is difficult to increase the number of the patients at late stage in a short time due to very rare of patients underwent surgery when diagnosed as advanced stage. As a complement, we reanalyzed the prognostic value of SOX30 in the patients at late stage after categorizing the patients as SOX30-high or -low group according to the median score of the patients at only stage III + IV (the number of high SOX30 expression patients at late stage is 40, and if SOX30 expression is associated with the survival of ADC patients at advanced stage, it should also be reflected in the regrouped patients base on the median score of only late stage patients). The result showed that SOX30 expression is still not associated with survival of the ADC patients at late stage (p = 0.718). Thus, SOX30 is indeed a favorable prognostic factor for ADC patients at early stage, but not for the patients at advanced stage. SOX30 as a tumor metastasis suppressor is a favorable prognostic marker for the ADC patients at early stage, indicating that SOX30 has acted important role in the tumor onset of early stage, which further shows the powerful role of SOX30 in ADC.
SOX30 has different prognostic values in ADC and SCC patients at early stage. From the data of our present study, the reason is that SOX30 plays different roles on tumor metastasis as differently direct regulation of CTNNB1. In early-stage ADC patients, the elevated SOX30 represses Wnt/CTNNB1-signaling by directly binding to CTNNB1 promoter to inhibit tumor-metastasis and results in a favorable prognosis of these patients. While in early-stage SCC patients, SOX30 has no role on tumor-metastasis and Wnt/CTNNB1-signaling duo to not binding to CTNNB1 promoter and results in an unfavorable prognosis of the patients. However, why SOX30 can not bind to CTNNB1 promoter in SCC patients is still unknown. Is there any mutation/deletion in the key binding sites of SOX30 binding to CTNNB1 promoter or whether the key binding sites of SOX30 binding to CTNNB1 promoter was occupied by other factors in SCC patient?., Further studies are required to clarify this interesting issue. Another remaining question is that SOX30 do not affect the tumor metastasis in SCC, how it can become an unfavorable prognostic factor for OS of SCC patients? Further studies are required to explain this problem.